The risk of developing a second cancer after early-stage breast cancer is low, according to a study of patients in England

Is the study of good quality?

"I believe so. The study has a robust design which, with a very large sample size and the use of data obtained from national population registries, gives it a high level of quality and reliability. Thanks to the inclusion of hundreds of thousands of patients and prolonged follow-up (several years), the results are statistically robust and accurately represent the real experience of women diagnosed with breast cancer in England. In addition, the statistical analyses used are rigorous and appropriate for the type of information being evaluated.

However, caution is advised: as with most observational studies based on health records, although techniques have been applied to minimise potential biases, the observational nature of the study does not allow definitive causal relationships to be established.

Is there a contrary belief among patients?

‘I don't think so; in general, patients eventually ask if they are at greater risk of cancer. That said, we must bear in mind that when a patient has cancer, what concerns her most at that moment is the tumour she is suffering from. The rest comes later. But it is true that, on some occasions, they do ask about it during consultations.’

The study talks about the risk of a second cancer, but not the risk of relapse. Is it important to differentiate between the two?

‘Very much so, and that's an excellent question. In breast cancer, we can have three situations: new breast cancer, local or regional relapse of the previous cancer, or the presence of metastases. The treatment and prognosis are clearly different, and it is essential to differentiate between them.’

What are its limitations?

‘As I said before. Caution: as with most observational studies based on health records, although techniques have been applied to minimise possible biases, the observational nature of the study does not allow definitive causal relationships to be established.’

What implications might this have for clinical practice?

‘I think that oncologists, in general, already know these things. However, this study provides very long-term data and also discusses other less common types of tumours. The most important thing is to discuss in detail the potential (tumour) side effects of treatments, look for different strategies (if any) and follow up reasonably.’

Overall, this is a high-quality study: a population cohort in England with 476,373 women with early breast cancer who underwent surgery, follow-up for up to 20 years, and analysis by age and treatment, published in The BMJ. The main finding is reassuring: the long-term risk of a second primary cancer is low—only ~2-3% above that of the general population—with absolute excesses at 20 years of +2.1% for non-breast tumours and +3.1% for contralateral breast cancer (more pronounced in young women).

In addition, the authors and a patient opinion piece they cite point out that many survivors tend to overestimate this risk, so these data help to correct that perception.

It is key to differentiate between second cancer and relapse: the former is a new, independent tumour; relapse is the return of the initial tumour, and this study only evaluates the former.

As for limitations, some registry data may be incomplete, and family history, genetic predisposition, and lifestyle habits (e.g., smoking) are not available, which may affect certain associations. Even so, the sample size allows for accurate estimates and suggests that a small portion of the excess could be related to adjuvant therapies (radiotherapy with contralateral/lung, hormone therapy with endometrium, and chemotherapy with leukaemia), although the benefits of these treatments far outweigh these risks.

In practice, this does not change indications. I believe it may be reassuring for patients with breast cancer currently undergoing treatment and provides a basis for more balanced advice, proportionate follow-up and reinforcement of standard age-based prevention and screening.